Effects Of Exogenous Organic Acids On Resistance Of Larix Olgensis In Simulative Mixed Pb And Cd Stressed Soils

With the rapid development of industrialization,the problems of environmental pollution become more outstanding.In the northeast of China,many waste hills and unreclaimed lands are in need of artificial afforestation.At these stand conditions,heavy metal stresses often exist together.In this paper,Larix olgensis seedlings was used as materials,the growth,physiological-biochemical indicators and the contents of Pb and Cd with exogenous organic acid treatments in mixed Pb and Cd stressed soils were determined.Then the influence of organic acids on the resistance of Larix olgensis seedlings were analyzed.The main conclusions are as follows:(1)Pb and Cd desorption in soils increased after organic acids treatments,and the desorption of Pb was larger than that of Cd.The promoting effect was citric acid>succinic acid>oxalic acid,and the best effect was at 10mmol/L citric acid.(2)Exogenous organic acids affected the forms of Pb,Cd in mixed stressed soils.The organic acids could change stable form of heavy metals into the most active exchangeable state,lowered concentration of Fe-Mn oxide bound Pb and Cd,increased exchangeable,carbonate bound Pb and Cd significantly.The best concentration was at 10mmol/L citric and oxalic acid.(3)Cd and Pb contents in both leaves and roots of Larix olgensis seedlings increased significantly,and heavily concentrated in roots.Generally,the best concentration to promote the accumulation of Cd and Pb was 10mmol/L of citric acid and oxalic acid.(4)Compared to Ck,mixed Pb and Cd stressed stress affected the growth and physiological-biochemical indicators of Larix olgensis seedlings,specific performances were as follows:seedling mortality rate increased,growth rate and biomass in root,stem and leaf decreased,and the inhibition of root biomass accumulation was the greatest.Content of MDA in leaves increased,activities of SOD,POD decreased,content of proline,soluble protein and chlorophyll all decreased.(5)After the treatments of exogenous oxalic acid,citric acid and succinic acid,the above indexes changed to the opposite direction,which indicated that exogenous organic acids could effectively relieve heavy metal toxicity to plants,and played an important role in improving the Larix olgensis tolerance and resistance to heavy metal stress,and repair the heavy metal polluted soil.(6)The effect of exogenous organic acids on growth and physiological and biochemical indexes varied with stress time,organic acid type and concentration.From the perspective of stress time,the orders of effect on MDA,activities of SOD,POD were all 30d>3d>10d;increasing growth rate of seedling height,basal diameter and biomass of root,stem and leaf were 30d>20d>10d;increasing soluble protein and proline contents were all 20d>10d>30d.From the perspective of organic species,in general,the orders of effects on the growth and physiological and biochemical indexes were citric acid>succinic acid>oxalic acid.From the perspective of organic acid concentration,the optimal concentrations of organic acids to reduce the heavy metal toxicity were 5mmol/L of Oxalic acid,5mmol/L of citric acid,1mmol/L of succinic acid,respectively.